挖掘机油缸活塞矫直断裂原因分析

Straightening fracture cause analysis of the hydraulic cylinder piston for excavator

某机械厂使用100 mm规格的45钢圆棒加工一种挖掘机油缸活塞,在加工过程中发生矫直断裂。对45钢圆棒和断裂样品进行了化学成分、低倍酸洗、断口、金相组织等项目分析。结果表明:用于加工活塞杆的45钢圆棒力学性能、化学成分符合标准要求,钢中夹杂物含量少,组织是正常的铁素体+珠光体。活塞杆矫直断裂为脆性断裂,断裂源位于工件表面,断口微观形貌为冰糖状脆性断口。断裂件经酸洗后发现纵向表面存在较多沿周向分布的裂纹及较深、较粗的车加工刀痕,金相分析结果表明裂纹为中频淬火过程中形成的沿晶裂纹,裂纹区域存在回火马氏体硬脆相组织。活塞杆发生矫直断裂的原因是工件矫直前已存在中频淬火形成的淬火沿晶裂纹,在矫直应力的作用下,裂纹沿基体扩展直至发生断裂。通过改进中频淬火和回火工艺,提高工件表面加工光洁度可消除表面淬火裂纹,后续产品未再发生矫直断裂。

The 45 round steel bar（100 mm specification） was used to produce a kind of hydraulic cylinder piston for excavator in one machinery factory. The straightening fracture occurred during the processing. The 45 round steel and fractured sample were analyzed by chemical composition,acid pickling test,fractograph and metallographic structure. The results showed that the mechanical properties and chemical composition of 45 round steel bar for producing piston rod could meet standard requirements. There were few inclusions in the steel,and the microstructure was normal ferrite＋pearlite. The straightening fracture of piston rod belonged to brittle fracture. The rupture source was located on the sample surface,and the microtopography was rock candy-shaped brittle fracture. After acid pickling test,many cracks along the circumferential direction as well as deep and thick lathing marks were found on the longitudinal surface of fractured piston rod. The metallographic analysis results indicated that the cracks were intergranular cracks which were formed in medium-frequency hardening process. The hard and brittle structures of tempered martensite existed in crack areas. The straightening fracture cause of the piston rod could be explained as follows：the hardening intergranular cracks,which had been formed in medium-frequency hardening process before straightening of workpiece,were expanded along the matrix until to fracture under the action of straightening stress. The surface hardening cracks could be eliminated by enhancing the degree of surface finish through improving the medium-frequency quenching and tempering processes. The straightening fracture was not observed in the subsequent products.